With the advent of high density optical recorders using CD recordable and CD RAM optical disk drives, the need to improve the positional accuracies (tighter focusing and tracking tolerances) and the quality of the written marks (smaller mark length deviation and mark width variation)is ever increasing. Such higher performance is needed to maintain a reasonable phase margin for the high density written data. As we enter the era of DVD, the improvement in the position and quality of written marks becomes even more critical.
The technique of "Direct Read During Write" (DRDW) has been used to determine if there is a tracking offset during write by comparing the DRDW signals with the wobble signal. Active correction of the tracking offset can be exercised via a servo loop once the track offset (error signal) is obtained.
The invention presented here proposes to obtain the tracking error signal during write by comparing the two diagonal signals out of the servo detector quad using the very beam that is also doing the writing. Since the signals we rely n are DRDW signals, we call this method the DRDW differential phase tracking method.
At least two current methods exist for differential phase tracking and they operate by reading individual marks fully formed already on the optical media. They are thus formulated as a read only method. The hereby proposed method, on the other hand, utilizes the fact that the marks are in the process of being formed, and the signature of the DRDW signals are significantly different from those of the marks fully formed already. Thus the proposed method is useful for write operation only. However, the merits of differential phase tracking, namely relative insensitivity to spot movement on the detector due to disk tilt or objective lens movement during tracking, is maintained.